1. Field of the Invention
The present invention relates to an active-matrix liquid crystal display unit, particularly to an active-matrix liquid crystal display unit suitable for driving a liquid crystal by a thin-film transistor (TFT).
2. Description of Related Art
In TFT driving systems of excellent image quality generally used as active-matrix liquid crystal display units, a largest demand is price cutting thereof. Thus, it is essential to reduce the cost of fabrication process of the TFT-LCD (liquid crystal display) unit to which amorphous silicon is applied, to cut down the number of fabrication processes and to improve the throughput and fabrication yield. Recently, for the purpose of improving the throughput in the fabrication line of liquid crystal display units, glass substrates having the sizes of 470 mm.times.370 mm from which four 10.4-inch panels can be taken, and 650 mm.times.550 mm from which six 12.1-inch panels can be taken. In this case, in order to reduce the weight of liquid crystal display units, the conventional thickness of the glass substrates of 1.1 mm has been reduced to 0.7 mm or below although the sizes of the glass substrate have been increased. Thus, in the fabrication process in which the glass substrates of increased size are used, there is a case of an occurrence of a bending in the glass substrate itself during a process of transferring this glass substrate. A stress of a thin film including a metal film for signal lines is one of factors for causing the bending of the glass substrate itself.
Further, not only the price cutting but also progress in a higher resolution and a larger diagonal screen are demanded in the liquid crystal display unit. Resistance of signal lines increases when the line length is increased or the line width is reduced, and payasific capacitance also increases along with an increase in the number of line cross sections and an increase in the number of transistors connected to the lines. When a display unit is structured in the state of an increased number of signal lines and parasific capacitance, a time constant to be determined by the product of the resistance of signal lines and the parasific capacitance increases, and the image quality is deteriorated due to a dull waveform of the driving pulse. Therefore, a metal material of low resistivity is required for the lines of a TFT-LCD unit.
Under the above circumstances, as a metal line material for a TFT-LCD unit, an Mo-W alloy of low resistance around 20 .mu..OMEGA.cm has been proposed, as described in the Japanese Patent Application Laid-open Publication No. 6-317814. When this alloy is used as the line material, a tapered section is formed at the edge of the line, without a formation of a protrusion such as a void at the line edge, by a dry etching method. This improves the step-coverage at the time of forming other film on this line.
Prior-art technique have had problems that the cost of material of a target to be used for a sputtering method for forming this alloy thin film is extremely expensive since the Mo-W alloy thin film is used as the line material, the throughput of the dry etching method is low, and a dry etching unit is expensive. Thus, it is difficult to lower the price of the liquid crystal display unit according to the prior-art technique.
Al or an Al alloy may be used as a line material considering its low resistivity and low price and being able to be processed by a wet etching method for high throughput. However, if Al or Al alloy is used as the line material, a hillock (protrusion) is generated by the heat applied during a fabrication process, with a resultant risk of an occurrence of a short-circuiting between lines. In order to use Al or Al alloy as the line material, it is necessary to anodize a film or to laminate the film with other metal of high melting point. This rather increases the process cost along with an increase in the number of processes.
Cr can be processed by the wet etching method without a risk of an occurrence of a hillock, and is excellent in its adhesiveness with other film and its lower contact resistance. Cr is used as a line material as described, for example, in the Japanese Patent Application Laid-open Publication No. 5-188397. However, Cr has a problem that the film stress is extremely high within the range of the normal sputtering conditions. If the film thickness is increased in order to lower the resistivity, the bending of the substrate becomes larger along with an increase in the film thickness, with a resultant risk that there arises a defect in the substrate during its transfer and the substrate is broken if the substrate has a hurt on it. The simply use of Cr as the line material is difficult to achieve a high resolution and a larger diagonal screen of the liquid crystal display unit. The use of Ne as an inert gas for the sputtering is also considered as disclosed in the Japanese Patent Application Laid-open Publication No. 6-67214. The mere use of Ne still has a problem that it is difficult to make the resistivity smaller or to make the film stress smaller.